1. Field of the Invention
The present invention generally relates to a blood pressure measuring apparatus and method, and more particularly to a blood pressure measuring apparatus and method for measuring blood vessel elasticity.
2. Description of the Related Art
Recently, as concern for health has increased, it has become possible for a person to measure the state of his/her health using various medical devices. As a result, a subject can measure their own bio-signals, such as blood pressure, heart rate, pulse and the like. Specifically, in order to prevent hypertension (high blood pressure) or the like, it is important to regularly check the state of the blood vessels. This can be accomplished by checking for diseases, such as atherosclerosis (thickening of artery walls).
It may be said that the blood vessel health may be indicated by arterial stiffness, i.e., how much the inner wall of an artery is thickened and how much its distensibility (elasticity) is lost due to the accumulation of foreign matter on its inner wall. Arterial stiffness may be estimated by measuring blood vessel elasticity. FIG. 1 illustrates a Young's modulus (a measure of stiffness) of an artery in relation to pressure, from which it can be seen that as pressure increases, the Young's modulus E increases exponentially. Since the physical meaning of E is blood vessel elasticity required for increasing a unit diameter, it can be seen that if the y-intercept is constant, force required to increase the diameter of the same artery increases as the slope of the graph becomes steeper. In other words, in FIG. 1, the steeper slope of the graph indicates higher arterial stiffness, and the lower blood vessel elasticity, wherein E may be typically expressed as follows:E=Eoexp(αP)  (1)
In Equation (1), E0 and α are constants, exp( ) stands for an exponential function, and P is internal pressure of a blood vessel, wherein if E is expressed in this exponential form, it may be said that the blood vessel elasticity according to pressure change is determined by the constant α. Since it is not easy to directly measure E or α of a human artery, blood vessel elasticity measuring methods using Pulse Wave Velocity (PWV) are widely used as blood vessel elasticity measuring methods.
In order to describe such a method using PWV, reference is made to FIG. 2. FIG. 2 illustrates a general relationship between Pulse Transit Time (PTT) and PWV. As shown in FIG. 2, the pulse transit time is defined by the time required for an arterial pulse to arrive at a peripheral point, such as a fingertip or a toe, starting from the heart, and the PWV is obtained by dividing the vascular length by the PIT. The PWV may be expressed as Equation (2) below, wherein since a function of Young's modulus E is employed, the PWV will generally indicate a higher value as the blood vessel elasticity decreases. In particular, the PWV in an aorta is generally known as an early detection indicator for aging of blood vessels, arteriosclerosis, hypertension, diabetes, hyperlipidemia, renal disease, and the like.
                    PWV        =                              Eh                          2              ⁢              ρ              ⁢                                                          ⁢              R                                                          (        2        )            
In Equation (2), R is blood vessel radius, h is blood vessel wall thickness, and p is blood density. In Equation (2), as the blood vessel wall thickness increases, the PWV also increases, from which it can be seen that the PWV may vary depending on blood pressure, blood vessel diameter, and blood vessel wall thickness.
As described above, the PWV varies depending on the internal pressure of an artery, and also depends on characteristics, such as blood vessel wall thickness, blood vessel radius, and blood density, which are varied from person to person. Therefore, unless the PWV is measured for the same blood vessel at the same blood pressure, it is impossible to evaluate the absolute blood vessel elasticity using the PWV. That is, through the measurement of PWV, it is impossible to determine that the blood vessel elasticity is high as compared to another person, it is only possible to merely confirm that the blood vessel elasticity is improving or worsening when the PWV is continuously measured for the same individual at the same blood pressure.